[APM] Telemetry: Report whether a given cluster has enabled the ML integration for APM

[roncohen]

Would be great to have telemetry to determine to what degree people have the ML integration enabled in APM UI. We could start with number of clusters that have it enabled. Second iteration could be to report how many services are in the cluster and how many have ML enabled.

Potential solution

• Check whether an ML index related to APM exists

cc also @jasonrhodes as you've been digging into telemetry across o11y recently.

[elasticmachine]

Pinging @elastic/apm-ui

[jasonrhodes]

Yeah so just to be clear, we should try to do this as a simple ES query that is performed inside a single telemetry "fetch" method, so that when the telemetry cluster makes its regular request for data, the query just reports back information about whether ML is enabled for the cluster or not. Because this is "cluster-level telemetry", there's no need to tie it to UI sessions at all.

[dgieselaar]

@roncohen should we close this? We're now reporting the number of ML jobs per cluster, but not yet per service. See #51612.

[roncohen]

Sounds good. Thanks 👍